Chemical reduction plating process



United States atent CHEMICAL REDUCTION PLATING PROCESS Robert A.Spaulding, Huntington Woods, Mich., assignor to General MotorsCorporation, Detroit, Mich, a corporation of Delaware No Drawing.Application December 3, 1953, Serial No. 396,082

9 Claims. (Cl. 117-130) This invention relates to improvements in thedeposition of nickel from chemical reduction plating solutions and moreparticularly to an improved plating bath, its control and maintenance.

In the past, nickel has been deposited from chemical reduction platingsolutions which generally comprise an aqueous solution of awater-soluble nickel salt, a reducing agent such as sodium hypophosphiteor potassium hypophosphite, and various additives. Prior platingsolutions, both acid and alkaline, generally have been employed attemperatures of about 90 C. or higher. In many instances, depending uponthe article to be coated, certain catalytic metals are employed toinitiate the oxidationr eduction plating solution.

One of the principal difliculties in prior chemical reduction platingoperations has been the accumulation in the plating solution ofundesirable by-products, chiefly alkali metal chlorides, solublephosphites and an acid, the particular acid depending on the source ofnickel employed. In the past it has been necessary to dispose ofchemical plating solutions after a relatively short period of usebecause of the presence of these undesirable by-products. Moreover,these reaction products reduce the speed of the plating reaction andcause pitting and roughness in the deposited nickel before it isotherwise necessary to dispose of the bath.

I have now discovered that the aforementioned difliculties can beeliminated and an improved'nickel deposit obtained by utilizing awater-insoluble nickel salt as a source of nickel and by employing anion exchange resin to remove accumulated phosphite impurities from thesolution. More particularly, the water-insoluble nickel saltscontemplated in the present invention are those adapted, when dissolvedin the plating solution, to neutralize the by-product acid and tothereby regulate the bath pH, thuseliminating the necessity of separateadditions of alkali to maintain the desired pH.

Examples of suitable water-insoluble nickel salts are: nickel oxy saltssuch as nickel monoxide, nickel peroxide, nickel sesquioxide, nickelousnickelic oxide, and nickel superoxide; hydroxy nickel salts such asnickelic hydroxide and nickelous hydroxide; and carbonates such asnickel carbonate and basic nickel carbonate. By employing one or more ofthese water-insoluble nickel salts as a source of nickel, either inreplenishing a conventional chemical reduction bath, or in original bathmakeup, not only is the desired nickel content provided, but also thebath pH is regulated by the alkaline radical liberated in the platingsolution and chloride by-products are not formed.

The practice of the present invention contemplates treating a platingsolution of the foregoing type with an In combination with the use of Ianion exchange material. the above-described type of water-insolublenickel salts substantially all of the reaction products detrimental tocontinuous plating are removed. Moreover, nickel and its reducing agent,the essential bath ingredients, are m ne us repl sh ice The anionexchange material to be used in any particular application will dependon a number of factors. In general, I prefer to employ an ion exchangematerial which is weakly basic so as to avoid removal of weakly acidicbath constituents. Suitable ion exchange materials include inorganicsubstances such as dolomite (calcium-magnesium-carbonate); syntheticinorganic materials such as heavy metal silicates; as well as variousorganic ion exchange resins including phenolic or phenolicderivative'type resins prepared by the polymerization of an aromaticamine, such as aniline or metaphenylen diamine and formaldehyde; or bythe reaction of a polyamine, phenol and formaldehyde. Other satisfactoryanion exchange resins include those formed by nitration and reduction ofcopolymers of styrene, divinylbenzene and/or other unsaturatedcompounds. Resins of the anion exchange type generally are characterizedby reactive hydroxy groupings or amine type groupings as follows:

R 1- Rail-R 1% RN-'H ('J=NR RAT-H Where R is an aromatic or alkylsubstituent group.

Examples of commercially available anion exchange resins are thefollowing:

Typical of a nickel reduction plating bath which may be regulated inoperation by the practice of the present invention is the followingcomposition where the quantities expressed are per liter of water:

Nickel chloride grams 30 Sodium hypophosphite do 10 Glycollic acidmilliliters 35 Sodium hydroxide grams 15 It will be understood, ofcourse, that the above bath composition is intended only as anillustration of a typical reduction plating bath employing aWater-soluble nickel salt and that the present invention is applicableto a wide range of bath compositions. For example, plating bathscontaining about 5 to 50 grams per liter of a water-soluble nickel saltsuch as nickel chloride, nickel acetate, nickel sulfate, etc.;approximately 5 to grams per liter of a reducing agent, such 'ashypophosphorous acid, sodium hypophosphite or other solublehypophosphites; and about 15 to 100 grams per liter of a buffer such asglycollic acid, sodium hydroxy acetate, sodium acetate, sodium citrate,etc. and/or other bath additives can similarly be replenished inaccordance with the present invention.

Although various chemical reactions occur during the plating process andthe bath replenishment, the exact interdependence of which is notclearly understood at present, the following reactions illustrategenerally how the water-insoluble nickel salts of the present inventionserve to replenish the nickel content and regulate the bath pH.

Employing nickel hydroxide, hydrochloric acid in the bath is neutralizedand nickel is liberated as follows:

2HCl+Ni(OI-I)2 NiClz+2HzO Using basic nickel carbonate:

As shown in these reactions, when a nickel salt of the type contemplatedin the present invention is added to the plating bath, a soluble nickelsalt is formed therein to replenish the nickel consumed in plating.Simultaneously, the hydrochloric acid or other acid which had beenaccumulating in the bath is neutralized.

By employing water-insoluble nickel salts, the necessity of makingfrequent bath analyses for nickel content is eliminated. Moreover, theplating solution is not contaminated with by-product alkali metalchlorides and, since no foreign alkali need be added to the bath toregulate its pH, accurate pH determinations are no longer necessary.

The ion exchange treatment in accordance with the present invention maybe better understood from a consideration of the following: An ionexchange resin, indicated as R(OH)2, is treated with a solution of asoluble hypophosphite or hypophosphorous acid as follows:

Used plating solution, contaminated with phosphite, is then passedthrough the resin treated according to Reaction 1 or 2: a I

(3) R(H2PO2)2+NaI-I2PO3 (used plating solution) RHPO3+(HaPO2+NaI-I2PO2)plating solution In many instances, instead of adding acid to thesolution as is indicated by Reaction 3, it is desirable to activate orregenerate only a portion of the ion exchange resin. Hence, in apreferred bath treatment the exchange reaction is as follows:

4 R(OH)2+R(H2PO2)z+NaH2POs (used plating solution) 2R(HPOs)+2NaH2PO2(plating solution) The ion exchange resin, now contaminated byphosphite, may be stripped with caustic soda generally as follows:

(5) R(H2PO3 2+2NaOH- R(OH) 2 ZNaHPOa (waste) Instead of caustic soda,-itwill be understood, of course, that the ion exchange material may bestripped or re; generated with sodium carbonate or ammonium hydrox ide,as well as other basic materials.

The following are illustrative of ion exchange treatment in accordancewith the present invention:

Example I A nickel reduction plating solution, having a pH of 4.82 andcontaining 110.8 g./l. of NazHPOs and 5.7 g./l. 0f NaHzPOz, was treatedwith 14.5 g./l. of hypophosphorous acid (in terms'of NaHzPOz). Theresultant solution was then passed through Permutit W anion exchangeresin which is an addition polymer having amine type active groupings.The effluent solution had a pH of 4.72 and contained 50.4 g./l. ofNaI-IPO: and 11.0 g./l. of NaHzPOz. Thus the phosphite content wasreduced from 110.8 g./l. to 50.4 g./l.

Example II A nickel reduction plating solution containing 70.6 g./l. ofNazHPOa and 9.0 g./l. of NaHzPOz was treated with sutficieuthypophosphorous acid to produce a hypophosphite content of 18.6 g./l.The resultant solution was then passed through Permutit 'W anionexchange resin. The efiluent solution contained 32.8 g./l. of NaHPOs and15.9 g./l. of NaHzPOz. Thus the phosphite content was reduced from 70.6g./1. to 32.8 g./l.

t will be understood, of course, that the ion exchange resin instead maybe treated with hypophosphorous acid, sodium hypophosphite or the likein a separate step, as shown in Reactions 1 and/ or 2, prior to passingthe plating solution through the resin.

In general, I have found that electroless nickel plating baths ofthetype contemplated herein operate most satisfactorily at a pH withinthe range of about 3.5 to a pH at which a nickel salt is precipitated, apreferred pH range being from about 5.0 to 5.5. In most instances,

an electroless nickel bath is operated most satisfactorily at atemperature of at least about F. although a temperature within the rangefrom approximately F. to 210 F. is preferred. Any chemically resistantmaterial such as stainless steel, glass, ceramic ware, rubber, orplastics such as polyethylene, saran, various phenolic and vinyl resinsmay be employed in the fabrication of containers for the ion exchangematerial. Although the temperature at which the ion exchange treatmentis conducted may be varied in particular applications, it is generallydesirable to employ as low a temperature as possible, preferably aboutroom temperature, in order to avoid possible chemical attack on the ionexchange material.

Since electroless nickel plating baths frequently contain ingredientsother than the source of nickel, and reduc-. ing agents, it is preferredto select an ion exchange resin which will not adsorb desirable bathconstituents, such, for example, as glycollic acid. In regenerating theion exchange resin as indicated in Reaction 3, I have found that it isdesirable to employ hypophosphorous acid in an amount equivalent toabout /5 the weight of sodium phosphite which is to be removed from theplating solution.

While the present invention has thus far been described in some detailas being applicable for the treatment of chemical reduction platingbaths employing water-soluble nickel salts, the present invention alsocontemplates the formation'of an improved chemical reduction platingbath utilizing a water-insoluble nickel salt as an initial ingredient.ture of a reducing agent and a water-insoluble nickel salt selected fromthe group consisting of oxy nickel salts, hydroxy nickel salts andnickel carbonates. In most instances it is desirable to also includeglycollic acid, sodium hydroxy acetate, sodium citrate and/ or sodiumacetate. A preferred bath of this type comprises nickel carbonate,sodium hypophosphite, and glycollic acid.

A bath of the above-mentioned type can be produced by dissolving thedesired water-insoluble nickel salt, or salts, in glycollic acid andthereafter adding the reducing agent. If sodium acetate, sodium citrateor sodium hydroxy acetate is used, the nickel salt may first bedissolved in hypophosphorous or phosphorous acid, enough acid being usedto obtain the desired initial bath pH.

The following is an example of a reduction plating bath utilizing awater-insoluble nickel salt as an initial ingredient. The quantitiesexpressed are per liter of water.

Nickel carbon grams" 15 Glycollic acid (70%) millimeters 35 Sodiumhypophosphite ..grams 10 Preferred pl-l 5.0

Such a bath comprises an aqueous mix-,

ampere stantially insoluble in water, even at elevated temperatures, theuse of these salts in the replenishment of conventional reduction nickelplating baths involves certain difliculties which I have succeeded inovercoming by the following method: A water-insoluble nickel salt isemployed in the form of a slurry formed by stirring the finely dividednickel salt into water. A filter, conventionally employed in chemicalreduction plating systems, is then coated with the finely dividedwater-insoluble nickel salt by passing the slurry into the filterintake. The used plating solution, before or after ion exchange, is thenpassed through the same filter whereby a quantity of the water-insolublenickel salt is dissolved from the filter in the plating bath. Inpractice, the amount dissolved from the filter media will beapproximately equivalent to the amount of nickel consumed in theplating. Hence, the plating bath will constantly replenish itself untilthe charge of the insoluble nickel salt has been consumed. Moreover,since the amount of nickel consumed in the reduction plating operationcorresponds to the amount of acid produced, the alkaline radicalliberated in solution by the gradual dissolution of the nickel salt willneutralize an equivalent amount of acid. Thus the pH will remainsubstantially constant.

In general, electrodes nickel reduction plating baths of the foregoingtype operate most satisfactorily at a pH within the range from about 3.5to a pH at which nickel hydroxide is precipitated. A preferred pH rangeis from 5.0 to 5.5. Plating baths embodying the present inventiongenerally should be operated at a relatively high temperature foroptimum results. I have found that the bath temperature in mostinstances should be at least 160 F. although a temperature within therange from about 190 F.2l0 F. is preferred.

It is to be understood that, although the invention has been describedwith specific reference to particular embodiments thereof, it is not tobe so limited since changes and alterations therein may be made whichare within the full intended scope of this invention as defined by theappended claims.

What is claimed is:

1. In a chemical reduction plating operation utilizing a plating bathcomprising an aqueous solution containing a water-soluble nickel saltand hypophosphite reducing agent, the improvement which consists ofmaintaining the desired nickel concentration in the bath by additionsthereto of a water-insoluble nickel compound adapted to regulate thebath pH, and simultaneously removing phosphite by-products formed duringplating by treating said bath with an anion exchange material.

2. In a chemical reduction plating operation utilizing a plating bathcomprising an aqueous solution containing a water-soluble nickel saltand hypophosphite reducing agent, the improvement which consists inmaintaining the desired nickel concentration in the bath by additions ofwater-insoluble nickel compound which is adapted to regulate the bathpH, and removing phosphite by-products formed in said bath duringplating by treating said bath with an anion exchange material, saidanion exchange material being capable of adsorbing phosphite ionswithout substantial adsorption of weakly acidic bath constituents.

3. In a chemical reduction nickel plating operation of the typeutilizing a plating bath comprising an aqueous solution containing awater-soluble nickel salt and a reducing agent selected from the groupconsisting of sodium hypophosphite, potassium hypophosphite andhypophosphorus acid, the improvement which'consists of adding to saidbath at least one water-insoluble nickel compound selected from thegroup consisting of oxy nickel compounds, hydroxy nickel compounds, andcarbonate nickel compounds, and removing phosphite ions which accumulateduring said plating operation by treating said bath with an anionexchange material.

4. In a chemical reduction nickel plating operation utilizing a platingbath comprising an aqueous solution containing a water-soluble nickelsalt and a reducing agent selected from the group consisting ofhypophosphorus acid, sodium hypophosphite, and potassium hypophosphite,the improvement which consists of adding to said bath at least onewater-insoluble nickel compound selected from the group consisting ofnickel oxide, nickel hydroxide, nickel carbonate and basic nickelcarbonate, and removing phosphite impurities accumulated during saidplating operation by treating said bath with an anion exchange resin,said resin being capable of adsorbing phosphite impurities withoutadsorbing weakly acidic bath constituents and being characterized byactive groupings selected from the class consisting of hydroxygroupings, hypophosphite groupings and amine groupings.

5. In a chemical reduction nickel plating operation utilizing a platingbath comprising an aqueous solution containing a water-soluble nickelsalt and a hypophosphite reducing agent, the improvement which consistsof adding to said bath at least one water-insoluble nickel compoundselected from the group consisting of nickel oxide, nickel hydroxide,nickel carbonate and basic nickel carbonate, and removing phosphiteimpurities accumulated during said plating operation by treating saidbath with an anion exchange resin, said resin being capable of adsorbingphosphite impurities without adsorbing weakly acidic bath constituentsand being characterized by active groupings selected from the classconsisting of hydroxy groupings, hypophosphite groupings and aminegroupings.

6. In a chemical reduction nickel plating process in the type utilizinga plating bath comprising an aqueous solution containing a water-solublenickel salt, a reducing agent selected from the group consisting ofsodium hypophosphite, hypophosphorous acid and potassium hypophosphite,and a bufier, said bath maintained at a pH generally within the range of3.5 to 6.0 and operated at a temperature of at least F., the improvementwhich consists of maintaining the desired nickel content in the bath,regulating the bath pH, and removing undesirable by-products by addingto said bath at least one waterinsoluble nickel compound selected fromthe group consisting of nickel oxide, nickel hydroxide, nickel carbonateand basic nickel carbonate and treating said plating bath with an anionexchange resin capable of adsorbing phosphite ions from said solutionand releasing a reducing agent in the solution.

7. In a chemical reduction nickel plating operation of the typeutilizing an aqueous bath containing a hypophosphite reducing agent, asoluble nickel compound and a water-insoluble nickel compound toreplenish the soluble nickel compound and regulate the pH, theimprovement which consists in removing phosphite from said bath andreplenishing the hypophosphite reducing agent during plating by treatingsaid bath With an anion exchange material which has been previouslytreated with a source of hypophosphite ions.

8. In the chemical reduction deposition of nickel utilizing an aqueoussolution of soluble nickel compound and a hypophosphite reducing agent,the improvement which consists of passing said solution through a filtercontaining a water-insoluble nickel compound to maintain the desirednickel content in solution and removing phosphite by-products formedduring plating by treating said solution with an anion exchange resin,said resin being characterized by active groupings selected from theclass consisting of hydroxy groupings, hypophosphite groupings and aminegroupings.

9. In a chemical reduction plating process utilizing a plating bathcomprising an aqueous solution containing a water-soluble nickelcompound and a hypophosphite reducing agent, the improvement whichconsists in maintaining the desired nickel concentration in the bath byaddition of water-insoluble nickel compound, maintaining the desiredhypophosphite concentration in the bath by addition of hypophosphorousacid, and removing phosphite formed in the bath during plating bytreating said bath with an anion exchange material. 7

References Cited in the file of this patent UNITED STATES PATENTSBrenner et a1. Dec. 5, 1950 8 OTHER REFERENCES Y Paulso'n et al.:Plating, vol. 40, No. 9, September 1953, pp. 1005-4009.

Sussman et 211.: Industrial and Engineering Chemistry, vol. 37, No. 7,July 1945, pp. 618-624.

1. IN A CHEMICAL REDUCTION PLATING OPERATION UTILIZING A PLATING BATHCOMPRISING AN AQUEOUS SOLUTION CONTAINING A WATER-SOLUBLE NICKEL SALTAND HYPOPHOSPHITE REDUCING AGENT, THE IMPROVEMENT WHICH CONSISTS OFMAINTAINING THE DESIRED NICKEL CONCENTRATION IN THE BATH BY ADDITIONSTHERETO OF A WATER-INSOLUBLE NICKEL COMPOUND ADAPTED TO REGULATE THEBATH PH, AND SIMULTANEOUSLY REMOVING PHOSPHITE BY-PRODUCTS FORMED DURINGPLATING BY TREATING SAID BATH WITH AN ANION EXCHANGE MATERIAL.